My Odyssey threw a belt and the new one doesn't fit

My 2002 Honda Odyssey's alternator/serpentine belt started making a whirring noise and then got thrown. It's mostly intact except one of the ribs has been stripped off. Not knowing why it would come off, I wiggled the tensioner a bit. It had a couple of millimeters of play so I figured it needed replaced. The new one has the same play and I was able to put the new belt on easily without even having to pull back the tensioner. I called O'Reilly's and verified the part number but I compared it to my old belt and it was a little bigger. I put the old belt (or what's left of it) on the pulleys and it was still way loose. I checked the alternator and a/c pulleys. Everything SEEMS good. Nothing wiggles and the pulleys spin.

Yes, I double checked the belt diagram. I've got it like this: (Not the same year but same diagram as far as I can tell)

My working theory is that the harmonic balancer (AKA crankshaft pulley) is worn down to the point that it is too small to hold the belt on. Here's a photo of that pulley. It measures at just under 6 inches in diameter. I've already spent $90 that I may or may not have needed to spend on this project. I'd rather not spend another $80 for nothing. Does this seem like the likely culprit? What else could it be?

If you havenít already figured it out, or if someone else happens to land here trying to figure out why itís still loose like I was, itís the upper alternator stay bracket that bolts to the the head and the alternator. Itís broken, and itís hard to see from the top. It costs about $40 from Honda, $15-25 on eBay, or if you happen to have a pick and pull salvage yard nearby you can get it for about $5-10.

I looked for for three days trying to see who had a similar problem, found this thread and still had no good answers. The alternator seem ok from above but when I pushed up from the bottom it moved up, Knowing it was supposed to be stationary led me to finally find the bracket broken in Two pieces